Conversion of wave motion into electrical energy



1942- a. B. BAUER 2,305,595

CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY Filed April 7, 1941 4 Sheets-Sheet 1 Dec. 22, 194 2. 3 B, B, BAUER 2,305,596

CONVERSION OF WAVE MO'I'IdN INTO ELECTRICAL ENERGY Filled April 7, 1941 4 Sheets-Sheet 2 25 42 Ida i'u/e ias vomse zec 10o E-e k1e2zg ycies ver oiecand. INVENTOR.

a. B. BAUER Dec. 22, 1942.

CONVERSION OF WAVE MOTION INTO ELECTRICAL ENERGY Filed April 7, 1941 4 Sheets-Sheet 3 B. B. BAUER Dec. 22, 1942.

CONVERSION OF WAVE IOTION INTO ELECTRICAL ENERGY Filed April 7, 1941 4 Sheets-Sheet 4 Qazzenlol Patented Dec. 22, 1942 CONVERSION or ways MOTION m'ro ELECTRICAL ENERGY Benjamin B. Bauer, Chicago, 111., assignor to S. N. Shure and Frances Shure, trustees, doing business as Shure Brothers, a partnership Application April 7, 1941, Serial No. 387,215

14 Claims.

This invention relates to apparatus for conversion of wave motion into electrical energy and the converse. More particularly it relates to instruments of unidirectional nature, i. e., in which the instrument is active preferentially in one direction'only, throughout an extensive range of 'frequencies, being relatively inoperative in other directions. This application constitutes a continuation-in-part of my co-pending application, Serial No. 232,439 for "Conversion of wave motion into electrical energy, etc., now Patent No. 2,237,- 298. Other 'copending applications which are also continuatlons-in-part of Serial No. 232,439 are Serial No. 387,216 entitled Conversion of wave motion into electrical energy, etc.," filed April 7, 1941; SerialNo. 387,217 entitled "Conversion of wave motion into electrical energy, etc., filed April 7, 1941; and Serial No. 387,438 entitled "Conversion of wave motion into electrical energy, etc.," filed April 8, 1941.

Unidirectional operation has previously been obtained in both the transmitting and receiving transducers through a combination of a unit having a nondirectional (circular) polar sensitivity pattern with one having a bidirectional (cosinelaw) polar sensitivity pattern. A combination of two such units causes the resulting polar sensitivity pattern to -be unidirectional (cardioid) in shape, and it has been applied extensively in the past to transmitting antennas, microphone apparatus, etc.- For this latter application, one of the units is commonly made to operate on the pressure component of the sound wave (pressure transducer) and the other upon the pressurediflerence of the sound wave (velocity transducer). Addition or cancellation of the voltages generated in each unit occurs depending upon whether the incidence of sound is from the front incidence) or from the rear (180 incidence) of the instrument. Obviously, the voltagesgenerated by both units for the 180 incidence should be substantially equal :and opposite in phasethroughout the frequency range in which the cancellation is desired, which because of inherent differences in construction and operating principle is a di-flicuit thing to obtain in microphones operating upon dissimilar components of the sound wave.

One important object of my invention is to provide a unidirectional transducer operating over a wide frequency range and comprising in part two transducing elunents'operating on the same component of the sound wave, thus doing any with the necessity of subtracting outputs of two transducing elements working on dissimilar components of the sound wave.

Another object is to provide a unidirectional transducer with marked unidirectional properties over the operating range of frequencies.

A further object is to obtain an instrument with unidirectional sensitivity pattern by the action of wave eifects at two points in a sound wave, using one transducer element only. Other objects of my invention will become apparent as this specificationiproceeds.

Figure 1 is a diagrammatic layout of generalized apparatus embodying my invention; Fig. 2, a vector diagram showing the voltage relationships for a zero degree incidence of sound; Fig. 3, a similar view to Fig. 2 but representing the incidence of sound; Fig. 4, a diagrammatic view of a specific embodiment comprehended within the diagram shown in Fig. 6; Fig. 9, a frequency response curve of the microphone shown in Fig. 6, the upper curve showing the front side response and the lower dotted line showing the decrease of re-.

spouse for the rear incidence sound; Fig, 10, a part sectional view of a unidirectional dynamic microphone; Fig.11, a front view of the same; Fig. 12, a diagrammatic view of the equivalent electrical circuit of the microphone shown in Fig. 10; Fig. 13, a cross sectional view of the unidirectional crystal microphone equipped with an duction and reception of sound waves in air, al-

though it will become-apparent to those skilled in the art that it may be equally applicable to wave phenomena in other media. The transducer element or elements employed may beeither of the reversible type, such as piezoelectric crystal, moving coil. moving armature or condemer type, or of the non-reversible type such as, for example, the carbon-type. The theory set forth herein is applicable to receiving apparatus,- such as loudspeakers, as well as to transmitting apparatus such as versible type are employed, one instrmnent could serve interchangeably, both as a transmitter and as a receiver.'- I

The natureofm'yinventionissuchthatitcan microphones. It transducers of the rel be best explained by reference to the following equivalent electrical networks and circuit equatins. Fig. l is a schematic representation of two electroacoustic transducers A and A, generating respectively voltages E and E, and the interconnecting electrical network C. The transducers, which may operate on any function of the sound wave whatsoever, are spaced by an effective acoustical distance d which in general should be smaller than, or comparable to, one-quarter wavelength of the highest frequency at which unidirectional action is desired, although it will be shown later that transducers may be constructed having unidirectional properties at frequencies higher than that specified above by virtue of diffraction and other wave effects. C is a generalized network shown in an equivalent ir section, composed of impedances Z0, Z1, Z2, and Z0, Z1, and Z2. The impedance Z3 is connected to the receiver B which may be an amplifier or any other receiving device. For simplicity, the internalimpedances of the transducers A and A are here considered negligible, although if this isnot the case the proper internal impedances should be inserted in the network in carrying out the rear 180 incidenceof sound, respectively.

The respective voltages generated by the trans? ducers A and A will be displaced in phase by an angle givenby the equation:

e is the phase angle between the voltages E and w is the expression Zr 1 is the frequency, cycles per second a is the angle of incidence of sound Cv is the velocity of the sound wave.

Applying circuit analysis to the equivalent circuit of Fig. 1, it may be shown that the voltage e delivered to the receiving apparatus, is given by the equation:

Z Z ar-( ar Z1 'i Z1 'i Z! Z1. a( z '2) a) (c 9 7.

It may be shown, furthermore, that the voltage drop across any branch in a network composed of linear elements, due to the action of two sources A and B connected at any two points, may always be expressed as the sum of the partial voltage drops due to each source acting alone. Thus,.for the network of Fig. 1, the portion of e due to E can be expressed as PE where P is the coefficient of E in Equation II divided by the denominator, and the portion of e due to E' can I be expressed as QE' where Q is the coefficient of E in Equation II divided by the denominator. It is seen therefore that the expression for the voltage delivered to the receiving apparatus can be indicated in the form:

Any expression involving network elements, having the function of P and Q in Equation III, is herein called the network factor.

To obtain unidirectional action, the voltage 6150 should become zero. Therefore, the condition to be met is:

and hence the relation between coefficients P and Q should be such that:

and the nature of the network components is to be chosen to substantially maintain this relation throughout the frequency range in which the unidirectional action is desired:

Equation V is perfectly general and may be applied to any unidirectional transducing system having two generators and an interconnecting network, delivering the translated energy to a receiver. For the particular case of the network of Fig. l, the values of network factors P and Q specified above may be inserted into, e. g., (V), giving the following relationship to be fulfilled:

1+ Before describing more specific embodiments of my invention, its operation will be further clarified by the following explanation made in reference to Figs. 2 and 3, which are vector diagrams representing the voltage relations for front and rear incidence of sound upon the instrument of Fig. 1.

For the purpose of explanation, it is assumed that voltages E and E, generated by the similar generators A and A, are of unequal magnitudes, although this is not necessarily the case. The voltage E0 is shown leading the voltage E'o through an angle 5 determined by Equation I, while the voltage Eiao is shown lagging behind Eiao by the same angle, since reversal of the direction of incidence brings about reversal of the relative phase positions of the generated voltages. The network factors P and --Q are shown of the same relative magnitudes and angular position as the rear incidence voltages E'iso and E1 0 respectively, as specified by Equation V.

The 0 (front-incidence) condition is shown in Fig. 2. The voltage E0 is operated upon the vector P to give the vector PEo which is the contribution of the generator A to the total output voltage. The voltage E'o is operated upon by vector Q giving the vector -QEo which is the contribution of the generator A to the total output voltage. QEo is added to PEo giving the resultant output voltage en.

The (180) rear incidence condition is show in Fig. 3. The voltage E180 is operated upon by the vector P giving the vector PEm which is the contribution of the generator A to the total output voltage. The voltage E'iao is operated upon by the vector Q giving the vector -QE'1ao which is the contribution of the generator A to the total output voltage. It should be noticed that for the rear incidence condition, the voltages PEIBU and QE'iao are out of phase and of equal magnitude, and hence when the latteris added to the former, the resulting total output voltage is'zero.

A specific example of network selection will is the same as that of Fig. 1, with the following element values:

Z no

- (VII) mined from Equation I, their ratio being therefore equal to'a unit vector at the angle Substituting this value-of angle into the right hand side of Equation .VI, and that of the network elements of VII into the left handside of Equation VI, it is evident that the desired re- I lationship is:

i-t L'cq wc'a' g 1w LC+jwCR Q The left hand member of the Equation VIII represents a quotient of two vectors, each of which may be made very nearly a vector operatingat an angle proportional to frequency, if the relationship between resistance, inductance and capacitance is such that:

since substituting these values into'Equation IX gives the following relation:

CIR/2 2 It should be observed that the numerator and I the denominator of Equation X are the major terms of the expansion for the cosine and the sine functions: hence, as long as:

tc'R' 1 and tcR 1 The Equation X may be rewritten The frequency term to drops out of this equation, and therefore the condition for unidirectivity will be obtained if at which d is not larger than one-quarter'the wavelength of sound; thus, if d is equal to approximately 1.5 cm., unidirectional action is obtaind for all frequencies up to approximately 5,000 cycles per second.

one electroacoustical transducer.

The type of polar directivity pattern obtained with the use of my invention depends upon the operational principle of the transducers A and A. This may be shown by solving the Equation V for Q and substituting into Equation III, which gives:

Since A and A are similar generators, the ratio of voltages E and E will be a vector K having constant magnitude and acting at the angle #10; therefore, the ratio of E and E'180 will be a vector K at an angle rpm); therefor-e:

The expression in parentheses of Equation XV, at frequencies for which d is small compared to one-quarter wavelength of sound; may be shown to approximately equal the algebraic sum of the angles s and #1180. Substituting the values of these angles given by Equation I,

If the character of the transducers A and A is such that the voltage generated is independent of the incidence of sound (pressure-operated or nondirectional transducers), the polar characteristic of the combination will be a cardioid of revolution expressed by the quantity in parentheses in Equation XVI. This polar characteristic is shown graphically in solid line in Fig. 5.

If the voltage E and E varies as the cosine of the angle of incidence, which will occur if transducers are of the bidirectional or velocity-type, then E'=E'o cos 0 and:

The quantity in brackets of Equation XVII represents the polar characteristic shown graphically in dotted lines in Fig. 5. It is seen, therefore, that combining two velocity-type transducers and the network described results in an electroaooustic transducing instrument of very marked unidirectional properties. It will be observed that my invention may make use of any two transducers operating on the same wave function, even if their transducing principles were dissimilar.

Instead of providing the electrical network directly at the output of the transducers, it is possible to first amplify these outputs with two independent amplifiers. and combine the outputs after the amplification. This procedure would be considered of the nature of an equivalent.

Instead of employing two transducers and an electrical network to obtain unidirectional operation, my invention makes such operation possible through modifying wave disturbances at two points in space by means of equivalent acoustical networks and impressing these disturbances upon Thus, the transducer is sensitive to one pressure component of a wave emanating from some source in the medium, which component acts through one of such points and is sensitive also to another pressure component of the wave, this latter component act ing through the other of such points. An embodiment of my invention employing this'alternative is shown in cross section and rear elevation in Figs. 6 and 7. The transducer assembly consists of a diaphragm 22 suitably supported in a casing 23 which also contains the piezoelectric crystal 2!. The forces developed by sound pressure at the diaphragm are transmitted to the (XIV) (XVI) crystal by means of a connecting member 25 and the electrical energy developed therein is received from the crystal by means of conductors 41 and 48. The front side of the diaphragm is provided with'an acoustical damping screen 26 constituted of a suitable wire-screen support having one or more thicknesses of cloth forming acoustical resistance and inertance. Between the diaphragm 22 and screen 26, there is a cavity 21 having an acoustical compliance C.

The casing 23 has a circular opening 28 which serves as a housing for the piezoelectric crystal and also forms part of the acoustical network. The housing 23, the back plate 29, and the diaphragm 22 provide a cavity 40 the crystal 24 not occupying the whole ofv the aperture in the housing- At the rear side of the case by means of screws 4| is held a cover 29, provided with ridge. 30, the proper spacing being obtained by adjustment against compression of spring 42. Thus, a narrow passage 3| is formed, having acoustical resistance and an inertance. P denotes the sound pressure at the outside of the damping screen; P denotes the sound pressure at the outside of the passage 3| The effective acoustical path between these pressures is called (1. I have found that, at frequencies of sound for which the diameter of the casing 23 is smaller than one-half wave-length, the pressures P and P are essentially equal and separated by a phase angle given by Equation I.

The equivalent electrical circuit of the transducer and its associated acoustical network appears in Fig. 8 in which R and L, and R" and L are the acoustical resistances and inertances of the screen 26 and the passage 3|, respectively; C and C are acoustical compliances of the cavities 21 and 40 respectively. Z3 is the impedance of the transducer element itself. As a simplifying assumption, the impedance Z3 is considered as formed by the capacitance C: corresponding to the stiffness of the crystal 24, and the reactions of the medium are neglected. The voltage e developed across 23 represents the resultant pressure upon the piezoelectric crystal. It may be observed that this equivalent circuit is entirely identical with that of Fig. 4, when the im-' pedance Z3 of Fig. 8 is perfectly general, as in Fig. 4. The complex impedance Z: in both fi ures includes the special case of a capacitance C3 as shown explicitly in Fig. 8.

In the commonly used system of electro-acoustic analogies pressure, volume velocity, inertance, acoustic capacitance and resistance are considered the analogies of voltage, current, inductance, capacitance and resistance respectively. This is because these acoustical quantities appear in the analysis of the behavior of acoustical circuits in precisely the way the corresponding electrical quantities appear in the analysis of analagous electrical circuits. The generalized circuit of Fig. 1 may therefore conveniently be used to illustrate the principle of operation of my invention in the special casein which a single transducer is used with acoustic means for modifying wave disturbances at two points in space to obtain unidirectional operation. In this case E and E represent the eifectiv'e sound pressures at the input to the two acoustic network branches separated by an effective acoustical distance d. In Figure 8 for example, E and E' represent the pressures p and p of Fig. 6. The

tive pressure on the transducer which results from the combined eifects of E and E altered by acoustical network factors corresponding to P and Q as discussed above. All of the equations previously derived therefor apply to this case when the appropriate acoustic network elements are substituted for the electrical ones. Thence, the acoustic capacitance, resistance and inertance terms are selected in reference to Equations IX and XIII to provide the unidirectional action desired. The terms R, L, and C due to screen 26 and cavity2'l are small compared with terms R, L and C due to passage 3| and cavity 40. hence the last term of the right hand side of Equation XIII will not have a great bearing upon the unidirectional action of the microphone. found that in some cases it is convenient to leave the damping screen 26 out altogether, and when this is done the constants R and C of the Equation XIII have to be readjusted slightly to compensate for disappearance of the last term. I have found that in a microphone with the casing 23 having a diameter of 6 cm. and the cavity 40 having a volume of 8 00., the effective distance d is 3.5 cm, and satisfactory operation is obtained when the passage 3| has a circumferential length of 10 cm., a radial length of 0.1 cm. and a thickness of 0.01 cm. These dimensions give an approximate acoustic capacitance C of 5.7 10- cm. per 'dyne and an approximateacoustic resistance of R of 18 acoustical ohms. Since C, R. and d are not calculable with good degree of accuracy in terms of the physical dimensions of the instrument, I prefer to calculate the approximate dimensions for these terms, and obtain the final values by adjusting the thickness of the passage 3| by means of the screw 4| until correct unidirectional action is obtained. 0bviously an alternative procedure would be to adjust instead the volume of the cavity 40 or the length of the distance d which could be done by provisions for adjustably altering the size of the case 23. v

The adjustment of the thickness of the passage 3| has a marked efiect on the directional property of the microphone at low frequencies and therefore the provision of screws 4| to permit variations in this adjustment is desirable. In the specific structure described above a cardioid unidirectional characteristic is obtained with a passage thickness of only 0.01 cm. When this passage is closed the sound wave has access only to the front of the diaphragm and the microphone functions as a conventional pressure operated microphone and is nondirectional at low frequencies.

In the case of a thin passage of the type here employed, the acoustic impedance, ZA, is given approximately by in which u is the viscosity coeflicient of the medium generalized network C is then an acoustical one and Z3 is the effective acoustic impedance of the Us the passage length in cms. in the direction of flow t is the passage thickness in cms.

L is the peripheral length of the passage p is the density of the medium in grams per cubic centimeter, and

w is 211' times the' frequency.

From this we note that the acoustic resistance varies inversely as the cube and the inertance inversely as the passa e thickness. At low t e;

I have and the phase shift is due almost entirely to R, C, R and C.

In Equation XII the left hand side, wd/Cv,

,represents the phase difference between E and E which results from the time required for the sound wave to travel the distance d. The right hand side w(C"R'-C'R) represents the difference between the phase shift the pressure p If we define the quantity is as given by Equation XVIII in the expression for k and using this value of k in the equation of the limacon the variation in directional response with passage thickness may be determined. The

response is zero for the values of 0' which make cos 0=--lc. When It equals one, that is when Equation XIII holds, a cardioid directional pattern results. The value of cos 0 then equals -1 when 0 equals 180 and there is no response from the'rear of the microphone. This is the directional characteristic usually desired and the one to which I have given detailed consideration above. A plot of some of the other directional characteristics the microphone is capable of giving are shown in Figure 18. As It is made less than one, the microphone maintains its unidirectional property, or preferential response to sound of 0 incidence, but a minor lobe corresponding to diminished response to sound of rear incidence-occurs. The two angles of zero response, which may be thought of as being coincident for the special case of k equals one, are

symmetrically disposed with respect to 180 in the second and third quadrants. In the limit when k equals zero. that is when R'C' equals RC the'null angles are 90 and 270 and the microphone becomes a cosine bidirectional type.

The polar pattern shown in Figure 14 represents the response of the instrument in a plane through the principal axis of the microphone. The surface of revolution generated by rotating the curve about this axis represents the three dimensional response of the device. From this we note that when k equals 1 the response is zero on the principal axis for sound of 180 incidence. As It is reduced the surface in which zero response occurs is a conical one, the internal solid angle of which increases as k is reduced until in the limit when k equals zero the surface becomes the plane of zero response in the resulting cosine bidirectional microphone. 'As the value of k is made to exceed one, the surface of zero response disappears although the microphone retains its unidirectional property until the limiting value of infinity is reached at which the microphone becomes nondirectional.

Since it depends on the external shape of the microphone, as discussed above, and R and C are determined by the screen 26, and the volume of the cavity 21 respectively, the value of k is most conveniently changed by altering the product R'C'. In the embodiment shown in Figure 6, k is varied by adjusting screws II which alter the thickness of the passage and thereby determines R'.

The resistance, R, of the screen, 26, is normally a small fraction of the resistance R, of the passage, 30, so that the sound wave has relatively free access to the front of the diaphragm. All unidirectional characteristics of the microphone between the nondirectional and the cosine bidirectional which are given by the equation for the limacon are obtained by varying is between zero and infinity. The value of infinity is obtained when the passage is closed. The value of zero is obtained when RC equals RC. Since the passage resistance varies inverse-' ly as the cube of the passage thickness, this latter adjustment is obtained with a thin passage even with low screen resistance. With a typical screen resistance in a structure of the type shown in Figure 6 a cardioid characteristic is obtained with a passage thickness of 0.01 cm. and a cosine bidirectional characteristic with a passage thickness of the order of 0.03 cm. so that all unidirectional characteristics are obtained by changing the passage thickness from zero to 0.03 cm. Because all of the desired di- ,rectional properties are obtainable with a very thin passage 30, if the screen, 26, is properly chosen, the left hand side of Equation VIII may be made proportional to frequency overa relatively wide frequency range even when the passage thickness is altered to give different directional characteristics.

Instead of obtaining resistance R by meansof the passage 3!, it is possible to substitute the cover'29 with a suitable foraminous supporting member such as a. wire-screen disc having a number of thicknesses of cloth or felt or similar porous material attached to. it, completely covering the opening 28, as shown more clearly in Fig. 13. The cloth illustrated is designated by the numeral I0. Fig. 13 is similar to Fig. 6 except as to the use of the cloth screen 10 in place of the narrow passage 3| By a suitable choice of the thickness and porosity of the material employed, the proper value of acoustic impedance may be obtained. Sometimes it is difiicult to select a material having the exact ratio of resistance to inertance specified in Equation IX; however, it is seen from Equation X that the squared terms are second-order terms in expansion for cosine function, and therefore the exact relationship between the inertance term L and the resistance and capacitance terms R and C is not a vital one in obtaining the unidirectional operation of the instrument at low frequencies, and reasonable departure therefrom will affect the unidirectional action but slightly. The important adjustment, however, is the one between the terms expressed in Equation XIII.

I have mentioned previously that the Equation XII is'valid up to frequencies at which (1 is not larger than approximately one-quarter wavelength of sound. This corresponds, for the instrument of Fig. 6, to a frequency of approximately 2500 cycles per second. It should not be assumed, however, that above said frequency the unidirectional action ceases, because above 2500 cycles per second, the instrument tends to' become highly unidirectional in favor of sounds arriving from the front because of diffraction and the so-called baffle-effect due to the size of the case 23. The unidirectional action is therefore obtained essentially throughout all of. the I important frequency range.

I have found that when a plane wave of constant intensity and varying frequency is impressed upon the front side of the instrument of Fig. 6, the resulting alternating force upon the crystal 24 is approximately proportional to frequency up to the frequency at which one-quarter wavelength equals the effective distance d, becoming approximately independent of frequency for frequencies at which one-quarter wavelength is larger than the effective distance d. Since the voltage developed in a crystal is proportional to the force applied, I found it useful to provide a compensating electrical network which would deliver an output voltage substantially independent of frequency at the receiving apparatus. This network consists of a parallel combination of a' condenser 45 and a resistance 44, both in series with a larger condenser 46; said network is connected across the terminals 41 and 48 of the crystal, the receiver 49 being connected across the larger condenser.

Fig. 9 shows the frequency response obtained with this microphone and electrical network for plane wave incident upon the front (upper curve) and the rear (lower curve) of the instrument, indicating the type of discrimination obtained at all frequencies. The polar directivity pattern is a cardioid shown in solid lines in Fig. 5.

It may be found convenient in many instances to provide the desired electrical compensation in the receiver 49. For'applications in which it is desired to give predominance to higher frequencie of sound, the compensating network may be entirely dispensed with.

Another embodiment of my invention is shown in the part sectional elevation in Fig. 10 and front elevation in Fig. 11. -A moving coil consisting of a circular bobbin 50 having a winding and a dome-shaped diaphragm or cover 52 is arranged to move in an air gap 53 of a magnetic structure, thereby transforming its mechanical motions into electrical energy which is received from the winding 5| by means of conductors 5|.

The magnetic structure consists of a cylindrical permanent magnet 54 provided at one pole with an internal circular pole piece 55, and having an external pole piece 56 connected to the other pole of the magnet by means of several connecting rods 51 which provide enough cross sectional area to conduct the magnetic flux, and do not appreciably interfere at the same time with the free access of the sound waves to the inner pole shoe 55. Associated with the magnetic structure, however not performing flux carrying functions, is acircular plate 59 which defines the cylindrical chamber 58 and also by means of shims Bil forms a narrow passage 60 which establishes communication between this chamber and the volume withinv the moving coil.

The coil is suspended on a suitable elastic suspension 6| which permits axial motions in the air gap. The external diameter of the inner pole piece 55 is slightly smaller than the internal diameter of the bobbin, thus forming a narrow passage 62 leading into the cavity 63 which is defined by the moving coil. The stiffness of the suspension 6| is low so that the coil assembly is resonated at a low frequency, preferably in the neighborhood of 60 cycles per second. The resonant effect is not very pronounced, however, because of the damping resulting from motion of air in the passage 62.

The equivalent electrical circuit of the instrument placed in a sound wave is given in Fig. 12. E is the equivalent of sound pressure upon the front of the diaphragm 52. E' is the equivalent of the pressure of the sound wave at the passage 62; Z: is acoustical impedance of the coil and its suspension; R and L, the resistance and inertance terms of the passage 82; Ca is theacoustic compliance of the cavity 63; Rb and Li), the acoustic impedance of the passage 60; Ch, the acoustical compliance of the chamber 58; as in the preceding embodiment, the reactions of the medium are neglected. The effective acoustical distance between the pressures E and E will be called d, and for frequencies at which this dis tance is less than a quarter wavelength of sound, E and E may be considered equal in magnitude and displaced by an angle given in Equation I.

Comparing structures illustrated in Fig. 6 and Fig. 10, it will be noticed that the latter is similar to the former with the screen 26 removed, since the impedance of the narrow passage 3i in Fig. 6 corresponds to that of the passage 52 in Fig. 10, and the compliance of the cavity 40 in Fig. 6 corresponds to the total compliance of the cavities 58 and 53 in Fig. 10. This similarity may be further seen by comparing the equivalent circuits of Fig. 8 and Fig. 12, the former with the impedances R, L, and C (corresponding to the clamping screen 26 and cavity 21 in Fig. 6) removed. Comparing these equivalent circuits, the series impedances R and L of Fig. 8 correspond with the impedances of the same calling in Fig. 12, and the capacity C of Fig. 8 corresponds to the total capacity of the condensers Ca and Ch in Fig. 12. Therefore, Equation XiII may be used to determine the correct proportion between the resistance and compliance units in the structure of Fig. 10, the last term of the right hand member of the equation being set equal to zero since the front screen is not used in this embodiment. However, in the moving coil structure of Fig. 10, it is not feasible to make the passage 62 narrow enough to obtain the correct ratio between resistance and inertance as expressed in Equation IX and this would affect adversely the unidirectional properties of the microphone in the middle range of audio frequencies. I have found that by subdividing the compliance element into two approximately equal parts shown as Ca and Ch interconnected by a series impedance having a resistance value Rb approximately three times the value of R and an inertance value Lb approximately equal to L, the unidirectivity Equation V1 is very closely satisfied throughout substantially allof the audio-frequency range.

I have found that in a microphone as illustrated in Fig. 10 having an external pole piece 55 approximately 3.8 cm. square and a moving coil approximately 2.8 cm. in diameter, the equivalent front-to-back distance dis approximately 2.5 cm., and with a total volume of cavities 58 and 63 of 6 cc., correct operation is obtained if the passage 62 has an axial length of 0.16 cm. and a thickness of 0.007 cm. These dimensions given an approximate acoustic value of Ca and Cb of 2.1 10- cm. per dyne each, R and Rh of 17.3 and 50 acoustical ohms respectively, L and Lb of .0015 gram per cmfi. These terms are not calculable with high degree of accuracy and I claim:

1. In a sound translating device, a moving bodyv to the phase change which takes place due to said waves traveling said distance in said medium throughout a substantial range of frequencies, and means for adjusting said network to make the ratio between said phase shift and said phase change substantially infinity.

2. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to said component a phase shift proportional to the phase change which takes place due to said waves traveling said distance in said medium throughout a substantial range of frequencies, and means for adjusting said network to make the ratio between said phase shift and said phase change substantially zero.

3. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to said component a phase shift proportional to the phase change which takes place due to said waves traveling said distance in said medium throughout a substantial range of frequencies,

.and means for altering the character of said network to make the ratio between said phase shift and said phase change either substantially,

infinity or substantially zero.

4. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, a casing enclosing said body and forming with said body a structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, and coupling means including an acoustical network imparting to said component a phase shift bearing a constant ratio to the phase change which takes place due to said waves traveling said distance in said medium throughout a substantial range of frequencies, said network being adjustable to make said ratio either unity infinity or zero.

5. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves v.to which said body-is sensitive, coupling means including an acoustical network imparting to another pressure component of said waves a phase shift, and coupling means including a second acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks forming with the phase change which takes place due'to said waves traveling said distance in said medium a ratio which is substantially constant over a range of frequencies.

6. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to another pressure component of said waves a phase shift, and coupling means including a second acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks being substantially'proportional to the phase change which takes place due to said waves traveling said distance in said medium over a range of frequencies,

'7. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to'another pressure component of said waves a phase shift, and coupling means including a second acoustical network imparting to said one component a phase shift, the difference in thephase shifts of said components due to said acoustical networks forming with the phase change which takes place due to said waves traveling said distance in said medium a ratio which is substantially constant over a range of frequencies and which is greater than one.

8. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to another pressure component of said waves a phase shift, and coupling means including a second acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks forming with the phase change which takes place due to said waves traveling said distance insaid medium a ratio which is substantially constant over a range of frequencies and which is less than one.

9. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emittedfrom a sound source in an elastic medium, structure which for waves of normal frontincidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling meansincluding an acoustical network imparting acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks forming with the phase change which takes place due to said waves traveling said distance in said medium a ratio which is substantially constant over a range of frequencies, and means foradjusting the relative characteristics of said networks to make said ratio either unity or substantially infinity.

10. In a sound translating device, a' moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to another pressure component of said waves a phase shift, coupling means including a second acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks forming with the phase change which takes place due to said waves traveling said distance in said medium a ratio which is substantially constant over a range of 12. In a sound translating device, means adapted to be acted upon by sound wave components one of which is shifted in phase with respect to the other, an acoustical phase-shifting network having an inlet spaced fromsaid means, said network shifting the phase of sound pressure at its inlet through an angle proportional to frequency, means cooperating with said firstmentioned means for translating the effects of said wave components into electrical energy,.and means for changing said network from a setting in which the device has an unidirectional space pattern to a setting in which the device has a bidirectional space pattern.

' 13. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, a casing supportfrequencies, and means for adjusting the relative characteristics of said network to make said ratio either unity or substantially zero.

11. In a sound translating device, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source in an elastic medium, structure which for waves of normal front incidence adds an equivalent acoustic-al distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to another. pressure; component of saidwaves a phase shift, coupling means including a second acoustical network imparting to said one component a phase shift, the difference in the phase shifts of said components due to said acoustical networks forming with the phase change which takes place due to said waves traveling said distance in said medium a ratio which is substantially constant over a range of frequencies, and means for adjusting the relative characteristics of said networks to make said ratio either unity substantially infinity or substantially zero.

ing said body and having a cavity, said body forming one of the walls of the cavity, said casing providing also a second cavity, said body forming also one of the walls of said second cavity, and communicating means between each of said cavities and the atmosphere,said means comprising principally a thin passage and being effective in conjunction with said cavities to shift the sound waves admitted through one of said cavities to greater extent than the sound waves admitted through the other of said cavities, said difierence in phase shift being substantially proportional to frequency at all wavelengths larger than the dimensions of said casing.

14. In a sound translating system, a moving body adapted to vibrate and convert its vibrations into electrical variations, said body being sensitive to sound waves emitted from a sound source.

acoustical distance to one pressure component of said waves to which said body is sensitive, coupling means including an acoustical network imparting to said component a phase shift bearing a constant ratio to the phase change which takes place .due to said waves traveling said distance in said medium throughout a substantial range of frequencies, and a compensating electrical network transmitting the electrical variations produced by said body, said electrical network presenting an impedance which increases with frequency and delivering at its output electrical variations the amplitude of which is substantially proportional to the amplitude of said sound waves and substantially independent of their frequency.

BENJAMIN B. BAUER.

Patent 110. 2,305,596.

'csmncsm- 'OF commcmxon.

December .22, 1&2.

mums B. sauna.

It is hereby certified that the name of the assignee in the above rum bered patent was erroneously descrihed-and specified as "S. N. Shure and I Frances Share, Trustees, doing business asShure Bro'thers a partnershifa' whereas said name should have been described 'and specified as -8. N. Shireand Frances Shure, 'Trustee, doing business .as shur'e Brothers, a partner-.

p", 88 shown by the record of assignments in this office; and that the said Letters Patent should be read with this correction thereinthat the same mayoontorm to the record or the case in the Patent Office;

Signed and sealed this 6th day or July, A. n. 191;

r Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

